A diagnostic ultrasonic imaging system for medical use may be utilized to form images of tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that are caused to travel into the body. Echoes from the tissues are received by the ultrasonic transducer element or elements and are converted into electrical signals. The electrical signals are amplified and used to form a cross sectional image of the tissues. Echographic examination is also employed outside of the medical field.
A linear array of transducer elements may be used for echographic examinations, but a two-dimensional array is superior in many applications. For example, a two-dimensional array can be focused electronically, so that the array can be fixed in one position during the examination process. Electronic focusing is achieved by delaying signals to selected transducer elements in the array. The phase correction improves resolution.
An area of concern in the fabrication of two-dimensional arrays of transducer elements is the method of achieving electrical interconnections to transducer elements that are not exposed at the sides of the array. U.S. Pat. No. 4,825,115 to Kawabe et al. describes a method of providing excitation energy to a center column of elements. Bonding wires may be attached to center elements, whereafter a backing layer is formed using molding techniques. As noted in the patent, the difficulty with this interconnection scheme is that as the distance between the transducer elements is reduced in order to improve resolution, the potential of two bonding wires shorting together is increased. Kawabe et al. teaches that a preferred interconnection scheme is one that uses L-shaped printed wiring boards having first legs that contact the transducer elements and having second legs that extend rearwardly along the spacing between adjacent columns of elements. The backing layer is molded between the second legs of the L-shaped printed wiring boards. While Kawabe et al. provides a significant improvement over prior interconnection schemes, the first legs of the printed circuit board remain in contact with the transducer elements, so as to provide a surface for reflecting wave energy.
The interconnection scheme becomes even more problematic if each transducer element is comprised of a multilayer lamination of piezoelectric layers. One advantage of the multilayer transducer element is that the electrical impedance of the element can be reduced. The layers can be electrically connected in parallel to reduce the impedance by a factor of the square of the number of layers. However, an increase in the number of layers requires an increase in the number of connections to each transducer element.
An interconnect scheme is described by Goldberg et al. in "Multi-Layer PZT Transducer Arrays for Improved Sensitivity," IEEE Ultrasonics Symposium, 1051-0117/92/0000-0551, pages 551-554, 1992. Each element includes a ground via that is midway along the width of one edge of the element and includes a signal via that is midway along the edge opposite to the ground via. Insulation gaps shield signal vias from ground electrodes and shield ground vias from signal electrodes. The approach taken by Goldberg et al. provides an improvement in the interconnect approach, but the improvement is obtained at a sacrifice of performance. In a general sense, the three-dimensional ultrasonic beam profile, i.e. beam intensity as a function of direction, is a two-dimensional Fourier transform of an element aperture. The hour-glass shaped transducer elements of Goldberg et al. cause the beam intensity to be substantially more asymmetrical than if the transducer elements were to remain in their original square configuration.
An object of the present invention is to provide an approach to electrical interconnections of electrode layers of a multilayer transducer array, wherein ultrasonic beam symmetry is substantially maintained. Another object is to provide a method for forming the two-dimensional transducer array.